1. Field of the Invention
The present invention relates to an air bag housing pad cover in which is housed an air bag to protect vehicle occupants. More particularly it relates to an air bag housing pad cover which, in case of crash, rapidly breaks and opens without obstruction to the inflation of the air bag. The present invention also relates to a method for producing such an air bag housing pad cover. The air bag housing pad cover is provided in a steering wheel, front instrument panel or front door trim of automobiles and other vehicles.
2. Description of the Prior Art
Air bag housing pad covers in general use are made of foamed polyurethane incorporated with reinforcement (such as net and cloth). However, this material is poor in productivity and yield because it requires a long time to embed reinforcement and involves difficulties in embedding reinforcement at a desired position.
There are air bag housing pad covers of a double-wall structure. One is made up of an outer layer of soft resin and an inner layer of hard resin which are integrally formed. The inner layer has a slit to facilitate opening. Another is made up of inner and outer layers of thermoplastic elastomer. The thermoplastic elastomer for the inner layer has a higher 100% tensile stress than that for the outer layer. The inner layer has a groove along which the pad breaks and also has a groove around which the pad swings open. The outer layer has a groove along which the pad breaks.
The above-mentioned air bag housing pad covers of double-wall structure have several disadvantages. The inner layer with a groove has to be thin to facilitate breaking. This leads to difficulties in molding. The outer layer also has to be thin to reduce weight. The reduced thickness makes the groove visible from outside, aggravating the appearance. Moreover, the double-wall structure requires that injection molding be carried out twice with two molds for the inner and outer layers. This leads to a longer production time and an increased production cost.
On the other hand, there have been proposed air bag housing pad covers different than mentioned above. They include one which is formed from an ester--or olefin--based thermoplastic elastomer by injection molding and has a groove to facilitate breaking. They also include one which is formed from a low-density material, such as a cellular material expanded by a blowing agent at the time of injection molding or a thermosetting resin (e.g., unsaturated polyester resin and epoxy resin) incorporated with a light filler (e.g., hollow glass balloons).
The conventional air bag housing pad covers mentioned above have their respective disadvantages. The one made from an ester--or olefin--based thermoplastic elastomer feels harsh because the material has to be hard to ensure firm mounting. The one made from an olefin-based thermoplastic elastomer is so brittle at low temperatures that it breaks even at thick-wall parts. The one made from a urethane-based thermoplastic elastomer is heavy (due to its higher density than other thermoplastic elastomers) and has such a great tensile strength and elongation that is needs a large amount of energy for its opening. The one made from a gas-blown material has a thin-wall part (1 mm or less) for the groove which lacks the cellular structure and hence has an undesirably high tensile strength and elongation for its opening.